Liquid (volume) conversions

What is this for?

Household volume measures appear constantly in outpatient counselling, compounding worksheets, and prescription quantity checks. Pharmacy practice uses rounded conventional equivalents (1 teaspoonful = 5 mL, 1 fluid ounce = 30 mL) rather than the exact SI values, because dosing devices are manufactured to those rounded volumes. This converter uses the practice conventions and flags where they differ from the exact figure.

How to use it

1. Enter the amount you have and pick its unit from the dropdown beside the input.
2. Choose the unit you want in the "Convert to" list.
3. The table below the result shows the same amount expressed in every other volume unit at once.

Worked example

A discharge prescription directs "2 tablespoonfuls twice daily for 10 days." How many millilitres per dose, and what total volume should be dispensed?

Answer: 2 tbsp = 30 mL per dose. Two doses daily for 10 days = 20 doses × 30 mL = 600 mL to dispense.

Clinical pearls & pitfalls

• Never counsel a patient to measure a dose with kitchen cutlery. Actual household teaspoons range from roughly 2.5 to 7 mL; always dispense a calibrated oral syringe or dosing cup.
• The abbreviations for teaspoonful and tablespoonful differ only by case (t vs T), which is a recognised source of three-fold dosing errors. ISMP recommends writing prescriptions in millilitres only.
• A "fluid ounce" (volume) and an "ounce" (weight) are different units. Converting between them requires the specific gravity of the liquid.

Assumptions & limitations

• Uses pharmacy-convention equivalents (1 fl oz = 30 mL) rather than exact SI values (29.5735 mL). Over large volumes this diverges: 1 gallon by convention is 3840 mL, but exactly 3785.41 mL — a 1.4% difference.
• Does not account for the specific gravity of the liquid; it converts volume to volume only.

References

• United States Pharmacopeia. General Chapter <1221> Teaspoon.
• Institute for Safe Medication Practices. ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations. link
• Yin HS, et al. Liquid medication errors and dosing tools: a randomized controlled experiment. Pediatrics. 2016;138(4):e20160357.

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